1. Field of the Invention
The present invention relates to a control system for a vehicle equipped with an automatic transmission system which is constructed having a torque converter and a stepped automatic transmission mechanism. More particularly, it relates to a control system which relieves gear shift shock in the gear shift operation of such an automatic transmission system.
2. Description of the Related Art
By way of example, a prior-art control system of the type specified above is disclosed in the official gazette of Japanese Patent Application Laid-open No. 263248/1988. The control system is furnished with a timer in order to control the pressure of working oil which is supplied to the gear shift clutch of a stepped automatic transmission mechanism under the gear shift operation thereof. Thus, when a predetermined time period has lapsed since the generation of a gear shift signal, this control system subjects the oil pressure to a correction control on the assumption that the transmission mechanism has actually started the shift operation.
Other examples intended to relieve the gear shift shock of a transmission mechanism are disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 254256/1988 and No. 4544/1989. In the former control system, when the revolution speed or r.p.m. (revolutions per minute) of the input shaft of the transmission mechanism has reached a predetermined r.p.m. value, the output of an engine is subjected to a correction control on the assumption that the transmission mechanism has actually started its shift operation. In the latter control system, the point of time at which the shift operation of the transmission mechanism has actually been started is determined on the basis of a change gear ratio which is the ratio between the r.p.m. values of the input shaft and output shaft of the transmission mechanism, whereupon the output of an engine is subjected to a correction control.
With the control system of Japanese Patent Application Laid-open No. 263248/1988, however, in a case where at least one of the r.p.m. of an engine, the running load of a vehicle, etc. has changed during the gear shift operation, a time period from the generation of the shift signal to the actual start of a mechanical gear shift or the duration of the shift operation changes. Therefore, the timing of the oil pressure correction control in the shift operation deviates, and the shock of the shift operation cannot be sufficiently reduced.
It is unsatisfactory to try to recognize the start of the shift operation on the basis of the change gear ratio of the transmission mechanism as in the control system of Japanese Patent Application Laid-open No. 4544/1989. More specifically, as illustrated in FIG. 9, the change of the change gear ratio is not readily discernible in the vicinity of a time t.sub.1 at which the transmission mechanism actually starts its mechanical gear shift, and the shift start is thus recognized much later (at a time corresponding to a point A). Likewise, in the control system of Japanese Patent Application Laid-open No. 254256/1988, the change of the r.p.m. of the input shaft of the transmission mechanism is not clearly apparent in the vicinity of the time t.sub.1, and the shift start is recognized much later.
As stated above, none of the prior-art techniques can precisely determine the time at which the transmission mechanism actually starts the gear shift. Therefore, the oil pressure control or engine output control of the transmission mechanism cannot be performed at an appropriate timing, so the shift shock cannot be sufficiently reduced.